1. Field of the Invention
The present invention relates to a power supply device for a motor vehicle which supplies electric power to electrical components in the motor vehicle. More particularly, the invention relates to a power supply device for a motor vehicle which detects a short and a disconnection of power supplying wires, and interrupts the supply of electric power to loads. Further, the present invention relates to a power supply device for a motor vehicle which supplies large current to auxiliary devices only when the feeding of large current is necessary. Furthermore, the present invention relates to a room lamp device for a motor vehicle for efficiently managing the power consumption by a room lamp in a motor vehicle.
2. Conventional Art
Recent motor vehicles are provided with many electric devices, such as wipers, power windows, door locks, lights, and electronic control units. A number of wires (wire harnesses), which connect a battery to the electric devices, are stretched out on the car body. When the car is running, the stretched out wires are vibrated. In this case, there is a danger that the covering or coverings of one or some wires is broken and the wire covered therewith is disconnected. Particularly, when the covering of the power supplying wire is broken, the exposed wire comes in contact with the car body (at ground potential). A short current flows, so that the covering burns up.
To avoid such a trouble, in a conventional technique, a fusible line 1 and fuses 2, as shown in FIG. 23, are inserted between a battery B and loads such as electrical devices. When a short occurs, a short current melts the fusible line or one of the fuses to shut off the short current.
Each of the fusible elements of the fusible line or the fuses is designed to have a heat capacity, which is selected such that a preset time elapses till temperature of the fusible element reaches a fusible temperature. With such a heat capacity, there is no case that the load is erroneously operated by a rush current or a spike current of the load.
In a rare short in which the exposed part of the wire comes in contact with the car body many times for a short time period, there is a possibility that the covering of the fuses burns up while the fusible line or the fuses are not molten.
In a vehicle, electric devices A1, A2, . . . (generally denoted as A), as shown in FIG. 20, are connected to a car-carried battery B for receiving electric power by which the electric devices are driven. If the battery B is connected to the electric devices A in the reversed polarities, the electric devices will be destroyed. To avoid this, a reverse-connection preventing circuit X is provided in each electric devices, as shown. The reverse-connection preventing circuit X contains a diode D, as shown in FIG. 21.
Unexamined Japanese Utility Model Publication 62-161543 discloses a technique in which a reverse-connection preventing circuit X is provided in the prestage of electric devices A as shown in FIG. 22. The reverse-connection preventing circuit X includes a relay R and a diode D. When an ignition interlocking switch S is turned on, a relay coil Rb is excited to close a relay contact Ra, so that a battery B is connected to electric devices A. When the battery B is connected in the reverse polarities, if the ignition interlocking switch S is turned on, current to the relay coil Rb is blocked by the diode D. The relay contact Ra remains opened. Therefore, the reverse voltage (current of the reverse polarity) is not applied (fed) from the battery B to the electric devices A.
In the battery connection circuit of FIG. 20, the reverse-connection preventing circuit X is provided in each electric devices A. In this respect, the battery connection circuit increases the cost to manufacture. In recent vehicles carrying an increased number of electric devices, the cost-increase problem is serious. In the battery connection circuit of FIG. 22, a single reverse-connection preventing circuit X is provided in the prestage of the electric devices A. Therefore, the FIG. 22 circuit is cheaper than the FIG. 20 circuit.
The battery connection circuits using the reverse-connection preventing circuit or circuits X function to only prevent the reverse voltage from being applied to each electric devices A. Therefore, if the polarities of the battery connected are reversed, it is necessary to disconnect the incorrectly connected battery B and to connect again the battery B in correct polarities.
As shown in FIG. 16, a room lamp 501 as an illumination device is installed in a motor vehicle. It is a common practice that the room lamp device 501 is mounted, together with the lamp switch 502, on the ceiling of the inside of the vehicle.
The room lamp device 501 contains a series circuit consisting of the lamp switch 502 and a room lamp 503, as shown in FIG. 17. One end of the series circuit is connected through a battery connection terminal BT to a battery B, while the other end thereof is connected through a door switch connection terminal DT to a door switch circuit 504. The door switch circuit 504 is earthed.
The lamp switch 502 includes three fixed contacts 505a to 505c and a movable terminal 506 that may be brought into contact with any of these contacts 505a to 505c. With the construction, the lamp switch 502 may put the room lamp device 501 in any of three lighting modes. The first contact 505a is connected to the door switch circuit 504. The door switch circuit 504 includes a parallel circuit consisting of door switches 504a to 504c respectively connected to the doors. These door switches 504a to 504c are interlocked with the doors. When one door is opened, the door switch interlocked with the opened door is turned on. Therefore, in a state that the lamp switch 502 is set at the position of the contact 505a, when one door is opened, the room lamp 503 is lit up.
The second contact 505b of the lamp switch 502 is connected to nothing. Accordingly, when the lamp switch 502 is set at the position of the second contact 505b, the room lamp 503 never lights up irrespective of the opening and closing of the doors. The third contact 505c is earthed. Accordingly, when the lamp switch 502 is set at the position of the third contact 505c, the room lamp 503 remains lit up irrespective of the opening and closing of the doors.
Another room lamp device is known which allows the light-up state of the room lamp 503 to last for a preset time after the door is closed.
A room illumination device for a motor vehicle, electrically expressed as shown in FIG. 18, is disclosed in Unexamined Japanese Utility Model Publication 60-15754.
The illumination device disclosed is generally made up of a charging circuit 510, a soft off circuit 511 and a timer circuit 512. The charging circuit 510 includes a transistor constant current circuit 513 and a door switch circuit 504. The door switch circuit 504 is connected to the constant current circuit 513. The constant current circuit 513 operates when the door switch circuit 504 is rendered on, and supplies a constant current to the soft off circuit 511 and the timer circuit 512.
The soft off circuit 511 includes a comparator circuit 514, a triangle wave generator 515, and a discharging circuit 516. As shown in FIG. 18, the triangle wave generator 515 is connected to the positive input terminal of the comparator circuit 514, and the discharging circuit 516 is connected to the negative input terminal thereof. The discharging circuit 516 includes a parallel circuit consisting of a capacitor and a resistor, and is connected to the charging circuit 510. With such a connection, the comparator circuit 514 receives a difference of the signals output from the discharging circuit 516 and the triangle wave generator 515. The output of the comparator circuit 514 is connected to a transistor switch circuit that is inserted between the first contact 505a of the lamp switch 502 and ground.
The timer circuit 512 includes a transistor timer based on a CR time constant circuit. The transistor of the timer circuit 512 is connected between the soft off circuit 511 and ground. The soft off circuit 511 may be started and stopped by turning on and off the transistor of the timer circuit 512.
In the room illumination device thus arranged, when a door is opened, the charging circuit 510 is put in an on state (I of FIG. 19). Then, the time constant capacitor of the timer circuit is charged. In turn, the timer circuit 512 is put in an on state, and the discharging circuit 516 of the soft off circuit 511 is charged. Further, the triangle wave generator 515 is operated. The output signal is input to the comparator circuit as shown in FIG. 19. Consequently, the comparator circuit 514 produces a light-up signal which in turn lights up the room lamp 503 (FIG. 19).
When the door is closed, the charging circuit 510 is put in an off state (II of FIG. 19), and the discharging circuit 516 starts its discharging operation (FIG. 19). The output signals of the discharging circuit 516 and the triangle wave generator 515 are applied to the comparator circuit 514, which in turn produces a pulse signal consisting of a series of pulses ranging for a time t1. The pulse width of those pulses becomes shorter as shown in FIG. 19. The illumination intensity of the lamp 503 gradually decreases with time. The timer circuit 512 is turned off at the termination of time t2, the soft off circuit 511 is turned off, and finally the room lamp 503 is lit off.
A driver, for example, often leaves the lamp switch on or the door incompletely closed. In this case, the car battery will be dead.
In the lamp device of FIG. 17 and the lamp device with the room lamp device 501 shown in FIG. 18, when the lamp switch 502 is set to the position of the third contact 505c, the room lamp 503 is left connected to the battery B. If a driver leaves the car in a state that the lamp switch 502 is set to that position and the engine is not started up, the battery will be excessively discharged.
Since the lamp switch 502 is interlocked with the door switch circuit 504, the same problem arises when the first contact 505a is closed (usually, the lamp device is used in this state). If a driver leaves his car in a state that the engine is not started up and the door is incompletely closed, the room lamp device 501 is left connected to the battery as in the above case. Accordingly, the battery will be excessively discharged.
The battery capacity of the car is usually 24 to 36 AH ampere-hours! which is able to start up the engine. For the power consumption of the room lamp, if 12 V and 10 W (=5 W.times.two lamps), current is nearly equal to 0.8 A. Usually, the battery is put in an excessive discharging state for 30 hours (=24 AH/0.8 A), but the battery loses its ability to start up the engine before 30 h is reached.